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Counting Coins to Count Rome’s Population

In ancient Rome from about 250 B.C. to 130 B.C., census figures fluctuated between about 150,000 and 400,000 citizens. But subsequent censuses, after about 30 B.C., had totals of four million to nearly six million. So did the later surveys overcount, or the earlier ones undercount?

That’s been a longstanding question among historians. Now two researchers have tried to resolve it, using an unusual batch of data: numbers of recovered coin hoards.

In times of unrest, someone who buries treasure to keep it safe might not survive to recover it. If these hoards are found later and can be accurately dated, they can serve as a proxy to help characterize certain periods of history.

“Coin hoards give you an objective way to measure the violence of any particular decade,” said Peter Turchin, a professor of ecology and evolutionary biology at the University of Connecticut. The more hoards, the more intense the upheaval.

In Europe, hundreds of coin hoards dating to Roman times have been found. Dr. Turchin and Walter Scheidel of Stanford University correlated the hoard data with significant events in Roman history. They found that hoarding increased in unstable times — during the Second Punic War in the third century B.C., for example.

Then they created a model to quantify the effect of the instability on population. As described in a paper in The Proceedings of the National Academy of Sciences, they found that until 130 B.C., the hoarding data correlated closely with the lower census figures — as hoarding (and unrest) increased, population counts went down.

The researchers then used the model to predict what the population would be in the first century B.C., given the known unrest of that period. They found that the model predicted population counts much more consistent with the lower figures than the higher figures. In fact, the model predicted declining population growth for decades after 100 B.C.

So why the much higher census figures from that time? One accepted reason is that Rome granted citizenship to allies elsewhere in Italy. But another, Dr. Turchin said, could be that the census takers started counting women and children.

A version of this article appears in print on October 13, 2009, on page D3 of the National edition with the headline: Counting Coins to Count Rome’s Population. Order Reprints|Today's Paper|Subscribe